There is a conventionally known discriminating method of kinds of optical discs based a difference in base material thicknesses of optical discs.
FIG. 7 shows such a conventional discriminating method of optical discs.
First, a discriminating method of optical discs disclosed in Japanese Patent Application Laid-open No.H8-287588 will be explained.
In this conventional technique, if an objective lens 1 shown in FIG. 7 is brought close to a CD 52 or a DVD 53, a sum signal 55 having a small amplitude based on light reflected from the disc is obtained at a position where laser beam comes into focus on a base material surface 54. Then, if the objective lens 1 is brought further close to the CD 52 or the DVD 53, another sum signal 57 having a large amplitude based on light reflected from the disc is obtained at a position where laser beam comes into focus on a signal surface 56. These sum signals 55 and 57 are compared with level 1, and differential time T1 between the instant when the base material surface 54 was detected and the instant when the signal surface 56 is detected is measured based on a binary type sum signal. The base material thickness of the optical disc is recognized depending upon the length of the measured differential time T1, and a kind of the optical disc is discriminated based on the recognized base material thickness.
Next, a discriminating method of optical discs disclosed in Japanese Patent Application Laid-open No.H11-149640 will be explained.
In this conventional technique, the following method is proposed so as to enhance the discriminating precision of discs.
That is, an S-shaped signal portion 58 in a focus error signal generated based on light reflected from the optical disc is compared with level 2 and level 3, thereby generating an S-shaped positive detection signal and an S-shaped negative detection signal, and detection differential time T2 between an S-shaped positive signal and an S-shaped negative signal is measured. The differential time T1 between the instant when the base material surface 54 was detected and the instant when the signal surface 56 is measured. Then, T1/T2 is calculated, thereby normalizing differential time between the instant when the base material surface 54 was detected and the instant when the signal surface 56 is detected. This conventional technique uses the normalized T1/T2 value, thereby eliminating influence of sensitive variation in a focus direction of an actuator.
When a surface of the optical disc is deflected, however, since the base material surface and the signal surface are vertically deflected, sum signals 61 based on light reflected from the base material surface and sum signals 62 based on light reflected from the signal surface are generated at a plurality of locations, and positions are varied in the longitudinal direction. For this reason, this conventional technique has a problem that the time can not be measured precisely and discrimination error of discs is generated. Further, there are problems that the sum signal 61 of the base material surface has a small amplitude, noise is erroneously detected as the sum signal 61 of the base material surface, time can not be measured precisely and discrimination error of discs is generated.
According to a discriminating method of optical discs disclosed in Japanese Patent Application Laid-open No.H11-149640, two signals, i.e., a focus error signal and a sum signal are required, time measuring means of T1, binary means of the focus error signal and binary means of the sum signal are required, and the structure becomes complicated.
Thereupon, it is an object of the present invention to precisely discriminate kinds of discs even when a focus position is varied by deflection of surface and a plurality of S-shapes of the focus error signal are generated.
It is another object of the invention to eliminate influence of variation in laser power and reflection ratio of optical discs, and to discriminate discs precisely.